Commit f00cdc6d authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

shmem: fix faulting into a hole while it's punched 



Trinity finds that mmap access to a hole while it's punched from shmem
can prevent the madvise(MADV_REMOVE) or fallocate(FALLOC_FL_PUNCH_HOLE)
from completing, until the reader chooses to stop; with the puncher's
hold on i_mutex locking out all other writers until it can complete.

It appears that the tmpfs fault path is too light in comparison with its
hole-punching path, lacking an i_data_sem to obstruct it; but we don't
want to slow down the common case.

Extend shmem_fallocate()'s existing range notification mechanism, so
shmem_fault() can refrain from faulting pages into the hole while it's
punched, waiting instead on i_mutex (when safe to sleep; or repeatedly
faulting when not).

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Reported-by: default avatarSasha Levin <sasha.levin@oracle.com>
Tested-by: default avatarSasha Levin <sasha.levin@oracle.com>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f72e7dcd

mm/shmem.c

+52 −4
Original line number Diff line number Diff line
@@ -80,11 +80,12 @@ static struct vfsmount *shm_mnt; 
#define SHORT_SYMLINK_LEN 128



/*

 * shmem_fallocate and shmem_writepage communicate via inode->i_private

 * (with i_mutex making sure that it has only one user at a time):

 * we would prefer not to enlarge the shmem inode just for that.

 * shmem_fallocate communicates with shmem_fault or shmem_writepage via

 * inode->i_private (with i_mutex making sure that it has only one user at

 * a time): we would prefer not to enlarge the shmem inode just for that.

 */

struct shmem_falloc {

	int	mode;		/* FALLOC_FL mode currently operating */

	pgoff_t start;		/* start of range currently being fallocated */

	pgoff_t next;		/* the next page offset to be fallocated */

	pgoff_t nr_falloced;	/* how many new pages have been fallocated */
@@ -759,6 +760,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) 
			spin_lock(&inode->i_lock);

			shmem_falloc = inode->i_private;

			if (shmem_falloc &&

			    !shmem_falloc->mode &&

			    index >= shmem_falloc->start &&

			    index < shmem_falloc->next)

				shmem_falloc->nr_unswapped++;
@@ -1233,6 +1235,44 @@ static int shmem_fault(struct vm_area_struct *vma, struct vm_fault *vmf) 
	int error;

	int ret = VM_FAULT_LOCKED;



	/*

	 * Trinity finds that probing a hole which tmpfs is punching can

	 * prevent the hole-punch from ever completing: which in turn

	 * locks writers out with its hold on i_mutex.  So refrain from

	 * faulting pages into the hole while it's being punched, and

	 * wait on i_mutex to be released if vmf->flags permits.

	 */

	if (unlikely(inode->i_private)) {

		struct shmem_falloc *shmem_falloc;



		spin_lock(&inode->i_lock);

		shmem_falloc = inode->i_private;

		if (!shmem_falloc ||

		    shmem_falloc->mode != FALLOC_FL_PUNCH_HOLE ||

		    vmf->pgoff < shmem_falloc->start ||

		    vmf->pgoff >= shmem_falloc->next)

			shmem_falloc = NULL;

		spin_unlock(&inode->i_lock);

		/*

		 * i_lock has protected us from taking shmem_falloc seriously

		 * once return from shmem_fallocate() went back up that stack.

		 * i_lock does not serialize with i_mutex at all, but it does

		 * not matter if sometimes we wait unnecessarily, or sometimes

		 * miss out on waiting: we just need to make those cases rare.

		 */

		if (shmem_falloc) {

			if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&

			   !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {

				up_read(&vma->vm_mm->mmap_sem);

				mutex_lock(&inode->i_mutex);

				mutex_unlock(&inode->i_mutex);

				return VM_FAULT_RETRY;

			}

			/* cond_resched? Leave that to GUP or return to user */

			return VM_FAULT_NOPAGE;

		}

	}



	error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret);

	if (error)

		return ((error == -ENOMEM) ? VM_FAULT_OOM : VM_FAULT_SIGBUS);
@@ -1729,18 +1769,26 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset, 


	mutex_lock(&inode->i_mutex);



	shmem_falloc.mode = mode & ~FALLOC_FL_KEEP_SIZE;



	if (mode & FALLOC_FL_PUNCH_HOLE) {

		struct address_space *mapping = file->f_mapping;

		loff_t unmap_start = round_up(offset, PAGE_SIZE);

		loff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;



		shmem_falloc.start = unmap_start >> PAGE_SHIFT;

		shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;

		spin_lock(&inode->i_lock);

		inode->i_private = &shmem_falloc;

		spin_unlock(&inode->i_lock);



		if ((u64)unmap_end > (u64)unmap_start)

			unmap_mapping_range(mapping, unmap_start,

					    1 + unmap_end - unmap_start, 0);

		shmem_truncate_range(inode, offset, offset + len - 1);

		/* No need to unmap again: hole-punching leaves COWed pages */

		error = 0;

		goto out;

		goto undone;

	}



	/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */

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